diff --git a/js/src/Makefile.in b/js/src/Makefile.in
index ca42e333361..9a6b546f559 100644
--- a/js/src/Makefile.in
+++ b/js/src/Makefile.in
@@ -65,7 +65,7 @@ INTERP_OPTIMIZER = -O3 -fstrict-aliasing
 endif
 else
 ifeq ($(OS_ARCH),SunOS)
-MODULE_OPTIMIZE_FLAGS = -O
+MODULE_OPTIMIZE_FLAGS = -xO4
 endif
 ifeq ($(OS_ARCH),WINNT)
 MODULE_OPTIMIZE_FLAGS = -O2 -GL